A wheel loader, which is one example of a work vehicle, has a boom supported in moving up and down on the vehicle body; a bucket rotatably mounted to the distal end of the boom; and boom cylinders and bucket cylinders for operation thereof. Through operation of the boom and the bucket, excavation, hauling, or loading of earth or sand, or other such works are performed.
In some cases, a wheel loader of this kind travels with earth, sand, or the like loaded into the bucket. Due to the large mass of the vehicle as a whole when earth, sand, or the like has been loaded in the bucket, the vehicle may experience considerable vibration during travel. Because of this, ride quality may be diminished, and the bucket is likely to spill its load.
Thus, a conventional vehicle of this type is furnished with a travel vibration suppressing device. This travel vibration suppressing device provides a state of communication between a cylinder for operating a work machine, such as a boom cylinder or the like (herein, an example of a boom cylinder shall be described) and an accumulator when the vehicle is in a state of travel. In so doing, vibration during vehicle travel can be absorbed by the accumulator, and transmission of vibration from the boom cylinder to the vehicle as a whole can be minimized.
On the other hand, during work such as excavation with the bucket, for example, all of the power of the boom cylinders must be directed to the bucket. Specifically, if the boom cylinders and the accumulator are in communication during work, the power of the boom cylinders will be absorbed by the accumulator, and will not be transmitted efficiently to the bucket. This causes a drop in work efficiency.
In the devices shown in Japanese Laid-open Patent Application No. 05-209422 and Japanese Laid-open Patent Application No. 2000-309953,, the boom cylinders and the accumulator are placed in communication or blocked, depending on vehicle speed. Specifically, when the vehicle speed is less than a given threshold value, a working state is determined to exist and the boom cylinders and the accumulator are blocked; or when the vehicle speed is equal to or greater than the threshold value, a traveling state is determined to exist and the two are placed in communication. In so doing, during work, the power of the boom cylinders can be efficiently directed to the bucket, while during travel vibration can be minimized through absorption by the accumulator.
In Japanese Laid-open Patent Application No. 05-209422,, the boom cylinders and the accumulator are placed in communication when the vehicle speed reaches 5, km/h, whereas the two are not blocked by the time that the vehicle speed falls to 4.5, km/h or below. In so doing, frequent repeated communication and blocking between the boom cylinders and the accumulator when the vehicle speed hovers around the threshold value can be prevented.